In callitrichids, females ovulate 1-20 days after parturition, known as post partum estrus (PPE). Although this period seems to be crucial to the reproductive success of the pair, since the female may become pregnant again while the infants re dependents, only a few studies have examined this phase. With this purpose we studied 8 heterosexual adult pairs of common marmosets, during the first six weeks after parturition. They were house in observation cages (2x1x2 m) located in an outside area at the Nucleo de Primatologia and submitted to natural conditions of humidity, temperature and light/dark cycle. The pairs were observed 3 times a week, in 30 min. sessions, between 8:00-11:00 a.m. The results showed the occurrence of PPE in 5 couples, monitored by plasma progesterone ELISA. The ANOVA test (Tukey) showed that the affiliative behaviors (female allogrooming by male, F=6.63; p=0.011 and male allogrroming by female F=5.81; p=0.018) were significantly different betw3een the two xcconditions (PPE and non-PPE) with higher values for the PPE group. Attmepts of copulation were also more frequent (F=4.61; p=).034) for PPE group. These data suggest 1) PPE did not necessarily occur in all females; 2) there is a strengthening of the reciprocal investment in those pairs where the ovulations occur, and 3) females in the PPE group might be more attractive to males.